On Mon, 22 Nov 2004, Jamie Jensen wrote:

> This is a basic question when looking at Crystal
> specs, related to temperature
>
> If the stability is rated say at 50ppm, is that to say
> that over the range it can vary up to +/- 0.005% of
> the nominal value?  Or am I interpreting that wrong?

Idelly it means most crystals in a batch bought will be within 50ppm of 
rated frequency at standard temperature in standard circuit. Temperature 
drift and circuit effects as well as long term drift and circuit influence 
must be accounted for separately. I have never seen figures for long term 
stability published but normally you can expect it to stay where it was 
when new within 25ppm or so over 1 year in a real life circuit (not 
thermostated, not potted, not hermetically closed), unless driven very 
hard (not burned-in crystals, normal 50ppm rated low cost crystals). But 
the average error will be smaller than 25ppm. It also depends a lot on 
initial material quality. Some crystals like to drift more than others 
(under the same conditions). 25ppm is about 1 minute per month in a clock 
context. Most unadjusted inexpensive quartz timepieces are within this 
range in my experience.

It would be interesting to comment on 50ppm being +/-25ppm or +/-50ppm 
(what is the exact meaning of 50ppm when written like this, unqualified).

Peter
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